fighting against an open source monopoly.

Of course you're right, a FOSS monopoly is nothing like a normal one. 
What Theo meant is there aren't many alternatives (if any) which leads to 
less pressure on the project, and subsequently to possibly worse code.

Look at KDE/Gnome - sure, duplication of efforts, but also sharing of 
ideas and competition/coopetition - which is good for both. GCC could 
probably use a competitor/coopetitor, which is the point he makes.